The present invention pertains to generating low quality steam containing dissolved solids for use in a thermal recovery process of crude oil and particularly to a cogeneration system for producing the low quality steam. Once of the most successful methods for recovering heavy crude oil has been the use of steam to heat the formation to reduce the viscosity of the oil and permit it to be pumped from the reservoir. Various processes for steam thermal recovery have been developed such as steam flood where steam is injected into one well to drive the crude oil to a second or production well. Another is a steam soaking method in which the steam is injected into one well for a time with the well then being shut in to permit the steam to heat the formation after which the well is produced to remove the crude oil. All of these methods require a large amount of steam that requires a corresponding large quantity of water. Of course, some water is recovered with the produced crude oil but a large quantity of water remains in the formation. Since a large number of heavy oil formations are located in areas where fresh water supplies are limited, the practice has developed of using brackish water containing a large quantity of dissolved salts for forming the steam. To prevent the salts from forming scale on the exchanger during the steam formation phase, it has been customary to use low quality steam, for example, 80-85% steam. The salts remain dissolved in the remaining 15-20% or water phase of the steam. For this system to operate satisfactorily, it has been necessary to design the heaters for producing the steam using a single continuous flow path for the water. This ensures that the quality of the steam is maintained at the desired level and at no point in the heater does the steam become substantially dry steam, which would cause depositing of the salts as scale on the heating surfaces.
The above described system is utilized extensively and all of the heaters are either fired with natural gas or oil. In recent years the price of natural gas and/or oil has increased to a level that seriously affects the economics of the thermal recovery process. In addition, various regulatory measures have been passed in an attempt to conserve natural gas for other uses than firing heaters. Thus, it has become desirable to look at alternate fuels for producing the steam for thermal recovery processes.
An alternate fuel that could be used to fire the heater is, of course, coal. While coal could be used, the present practice of a large number of relatively small heaters does not lend itself to coal firing. Further, efficiencies that could be achieved with coal firing of the present heaters would be low compared to what can be achieved in large central power plants.
Central power plants can be designed to burn coal and achieve a high efficiency. Also cogeneration plants have been developed that provide electricity and steam for use in various processes while maintaining a high efficiency. The process steam is high quality steam that is condensed and can be used as feed water for the boiler. This is not possible in thermal recovery systems since the water used to form the steam is brackish and would quickly foul the high efficiency boilers. Thus, heat exchangers must be used to generate the thermal recovery steam.
A suitable heat exchanger is described in co-pending application Ser. No. 494,145 entitled "Once-Through Steam Generator". The application describes a heat exchanger having multiple, continuous flow paths between the inlet and outlet. This heat exchanger design allows the use of brackish water but is difficult to build. The exchanger requires U-bends at the end of each tube pass to form a continuous flow path. This adds to the complications and cost of building the heat exchanger.